The present invention relates to a method of driving a solid-state image pickup device used in an image capturing apparatus, such as a digital camera or a movie camera. The invention also relates to an image capturing apparatus using such a solid-state image pickup device.
In place of a conventional image pickup device dedicated to capturing moving images, an image pickup device that is capable of both capturing moving images and capturing still images is being more commonly used in an image capturing apparatus, such as a so-called electronic still camera or a movie camera. An image pickup device which is capable of and normally used for both capturing moving images and capturing still images is a so-called multi-pixel image pickup device capable of picking up a tremendously large number of pixels compared to a conventional image pickup device dedicated to capturing moving images, because a multi-pixel image pickup device is capable of capturing still images at extremely high resolution.
Using such an multi-pixel image pickup device to capture a still image produces a picture of superb quality, which is much better than a still image captured by a conventional image pickup device dedicated to capturing moving images; for example, the image is of such quality that it can be amply be appreciated even if printed in A4 size. On the other hand, a multi-pixel image pickup device has too many pixels to be used for capturing moving images without processing; it takes a long time to read out images contained in each frame of a movie, making it difficult to obtain smooth moving images. This can be solved by increasing the speed of the image pickup device itself or the peripheral circuits of the image pickup device. Such an increase in speed, however, would result in a drastic increase in cost and is therefore impractical.
Therefore, a multi-pixel image pickup device used for this purpose normally has such a structure that calls for individually reading out all the pixel signals to shoot still images, and omitting a part of the pixel signals to shoot moving images. In cases where a so-called CCD is used as the solid-state image pickup devices, omission readout is usually performed according to a vertical method, which basically calls for omitting some of horizontal scan lines in a given proportion. The CCD mentioned above is a CCD (charge-coupled device) image sensor comprised of a multi-pixel CCD image pickup device.
Omission readout is also performed for using an image pickup device itself as a sensor to measure conditions of image capturing, in other words to control the shooting system, such as preliminary measurement of shooting conditions conducted prior to shooting still images or moving images.
A CCD is an image pickup device having an image pickup surface comprised of photodiodes, which correspond to pixels and are arranged in a two-dimensional grid. An image focused on the image pickup surface by means of an optical system is retrieved by reading out signals representing amplitudes of electric charges that have been charged on the photodiodes according to the intensity of the light of the respective pixels. Signal charges are sequentially sent one signal charge for each pixel at a time to an output circuit in the CCD. The CCD includes a charge-voltage converter. After the amount of each charge signal is converted to a signal to change voltage, the signal is output from the CCD. This operation of guiding signal charges corresponding to pixels into the output circuit is called transfer. A commonly employed method of sequentially transferring signal charges of pixels arranged in a two-dimensional rectangular grid into an output circuit calls for a combination of so-called vertical transfer and horizontal transfer. Given that an alignment from the top to the bottom or vice versa of the image capturing frame is vertical and that an alignment from the right to the left or vice versa of the image capturing frame is horizontal, vertical transfer refers to operation of transferring the signal charges corresponding to all the pixels simultaneously in the vertical direction towards the aforementioned output circuit, and horizontal transfer refers to operation of transferring, simultaneously in the horizontal direction towards the output circuit, the signal charges that have been transferred to the horizontal transfer path from the front ends of the vertical transfer paths as a result of vertical transfer by the amount of one stage, said front ends being the ends that are closest to the output circuit and collectively form a horizontal line. At that time, the furthermost signal charge that corresponds to a pixel and is located in the horizontal transfer path, i.e. closest to the output circuit, is sent into the charge-voltage converter. Horizontal transfer continues in this manner until all the signal charges in the horizontal transfer path are converted to voltages. When the horizontal transfer path becomes empty with completion of the output, the aforementioned vertical transfer is again performed. After the new signal charges on the horizontal line that form the furthermost ends of the vertical lines of signals are sent into the horizontal transfer path, the signal charges are sequentially transferred into the output circuit by horizontal transfer in the same manner as above and output from said output circuit. By thus repeating simultaneous vertical transfer of the signal charges of all the pixels and horizontal transfer of the signal charges in the horizontal transfer path alternately until the signals of all the pixels are output, output of pixel signals for one frame is completed. Thus, an electronic image can be reproduced.
When each signal charges of the pixels are transferred, vertical transfer is normally performed in the forward direction, i.e. toward the horizontal transfer path, as described above. Because of the principles of CCD, however, it is possible to perform reverse vertical transfer, in other words in the direction away from the horizontal transfer path. In response to the recent trend of reducing power consumption as well as pixel minimization resulting from increase in the number of pixels, progress is being made to reduce the transfer driving voltage of CCD. The reduction of the driving voltage is complicating efficient vertical transfer, a process which had conventionally been simple. Under such circumstances, some of the multi-pixel CCDS are designed to achieve high efficiency in vertical transfer in the forward direction. In some cases, however, this has resulted in drastic decrease in the efficiency of reverse transfer.
Among CCDs of various types, those most commonly used as the multi-pixel CCDs are what are generally called as interline-type CCDs. Depending on the scanning method, the interline-type CCDs are divided broadly into CCDs using interlace scanning methods (hereinafter called interlace CCDs) and CCDs using progressive scanning methods (hereinafter called progressive CCDs). At present, interlace CCDs are more widely used, because they achieve the most effective cost performance and have a structure suitable for multi-pixel image pickup devices. According to the structure of a typical interlace CCD, the number of horizontal lines of signal charges (otherwise referred to as pixel signal charges, charges, or pixel charges) that can be retained in the vertical transfer paths is a half of the number of the horizontal lines of photodiodes that serve as a photoelectric converting unit and correspond to the respective pixels. Therefore, in order to read out all the pixels in a frame respectively, the frame is divided into two fields, and readout is performed twice: even-numbered lines and odd-numbered lines alternately, in other words in an interlacing manner. For this purpose, two discharge readout electrode systems (otherwise referred to as signal discharge readout electrode systems or discharge read gate electrode systems) are respectively provided for the even-numbered lines and the odd-numbered lines. With interlace CCDs, selective readout may be enabled; for example, the number of the discharge readout electrode systems may be increased from two to four to permit a part of the lines to be selectively read out to the vertical transfer paths so that all the combinations of color filters in a single field are read out while the other lines remain unread. Thus, the function of line-omission is easily obtained. As a multi-pixel interlace CCD of this type normally call for 4-phase drive to perform vertical transfer, it is necessary to increase the number of electrode systems for the vertical transfer paths from the conventional 4 electrode systems to, for example, 6 electrode systems including the 4 discharge readout electrode systems.
Unlike interlace CCDs described above, progressive CCDs are capable of sequential readout of all the lines in a frame from the frontmost line, because a number of signal charges corresponding to the number of lines of photodiodes can be retained in each vertical transfer path. If omission readout is not to be performed, a single charge readout electrode system is sufficient. However, in cases where the function of omission readout is desired, another charge readout electrode system has to be added to enable the selective readout of lines. Therefore, including the total of two charge readout electrode systems, a progressive CCD requires an increase of the electrode systems for vertical transfer from the conventional 4 electrode systems to 5 electrode systems in cases of 4-phase drive, or from the conventional 3 electrode systems to 4 electrode systems in cases of 3-phase drive.
An increase in the number of charge readout electrodes complicates the wiring inside the CCD in proportion to the increase in the number of the electrode systems or increases the external CCD-drive circuits. In spite of these drawbacks, however, most multi-pixel CCDs have structures that call for a large number of electrode systems. One of the reasons is that there is no alternative method that may offer a lower cost.
There are conventionally known examples of image capturing apparatuses which are capable of switching between two modes: a full-pixel individual readout mode for reading the signal charge of each pixel and using the signal charges of all the pixels on the CCD in the manner described above, and an omission readout mode for selectively reading out the signal charges of only a part of the pixels on the CCD and abandoning the signal charges of the other pixels without using them. To be more specific, the full-pixel individual readout mode is used as a still-image mode principally intended for shooting still images, and the omission readout mode is used in cases that require driving at a high frame rate, such as shooting moving images, or for preliminary measurement, which is performed prior to a main shooting.
In addition to usual line-omission readout explained above, various methods of horizontal line-omission or line-summation readout have been offered for progressive CCDs, including one disclosed in Japanese Patent Provisional Publication No. 10-136244. These methods are broadly divided into three categories: (1) methods that call for reading out only ‘n’ number of lines out of ‘m’ number of lines (m>n, m≧3), (2) methods that call for summation of signal charges of ‘n’ number of lines out of ‘m’ number of lines to perform readout (m>n), and (3) methods that call for summation of signal charges of ‘q’ number of vertically extending lines. The aforementioned Japanese Patent Provisional Publication No. 10-136244 discloses a structure that is different from methods of simply omitting unwanted lines in that it is capable of improving the frame rate while it calls for summation of signal charges of a plurality of lines. Another example is disclosed in Japanese Patent Provisional Publication No. 10-210367, which relates to a structure that calls for individually reading and using the signal charges of all the pixels on the CCD to shoot a still image, and either omitting or summing up a part of the pixels on the CCD when shooting a moving image or a still image.
As described above, conventional methods that call for selectively reading out specific lines while omitting the other lines present the problem of reduced sensitivity, although they are capable of improving the frame rate. In other words, shooting a moving image in the omission readout mode in low-light conditions results in an image with a poor S/N (signal-to-noise ratio) quality due to insufficient sensitivity. When performing preliminary measurements such as photometry or focusing prior to a main shooting, too, the insufficient sensitivity causes drastic reduction in the resolution. Compensating for the insufficient sensitivity with emitting auxiliary light to perform preliminary measurements necessitates the use of a costly, bulky, very bright lamp, which requires a lot of electric power. Furthermore, when the sensitivity is not sufficiently high, it is not possible to reduce the exposure time for preliminary measurement while the measurement precision remains low. This may result in a considerably long lag in time, which is the time required from the moment the user intends to shoot to the moment exposure for the main shooting starts.
During shooting moving images, merely omitting lines may reduce the spatial frequency reproducibility and generate moiré, largely degrading the image quality. To be more specific, in the state where lines have been omitted, MTF (modulation transfer function) of the photographing lens remains unchanged, although the vertical spatial sampling frequency and the proportion of the opening have been reduced in proportion to the omitted lines. As a result, a conspicuous reflected distortion is generated, producing moiré, which is a phenomenon that may cause, for example, a subject having a pattern of closely spaced apart stripes to appear in coarse, large stripes—in other words, something quite different from the actual subject. This is an undesirable phenomenon for an image capturing apparatus.
As disclosed in Japanese Patent Provisional Publication No. 10-136244, one of the solutions to ameliorate this problem is to employ line-summation instead of merely omitting lines to improve the frame rate. Even when MTF is high in the state of the reduced spatial sampling frequency after line-omission, line-summation brings about the same effect as that of spatial filter processing by increasing the proportion of the opening and reducing high-range spatial sampling frequency components.
Line-summation can be performed most effectively by, for example, summation of five each lines of color filters of the same color to achieve a result that is equivalent to that which could be achieved by reading out all the pixels at a frame rate that is five times faster, instead of using a conventional omission readout method which calls for reading out only one-fifth of all the lines and omitting the remaining four-fifths to achieve a 5-fold increase in the frame rate. As the spatial filter is at its most effective while the signal charges of all the pixels contribute to image capturing, the proportion of the opening becomes exactly the same as that for still-image capturing.
However, Japanese Patent Provisional Publication No. 10-136244 does not provide a method of reading out the signal charges of all the pixels by reading out the signal charges of all the pixels by summation of signal charges on a plurality of lines of a like color in such a manner as described above. Although a process of full-pixel reading by summing up signal charges of lines that are adjacent to one another on either the vertical transfer paths or the horizontal transfer path is mentioned in the above patent document, such a process will cause an undesirable mixing of colors in cases of color CCDs, except for those having a vertical stripe filter arrangement. Such an undesirable mixing of colors not only greatly reduces the color reproducibility but, depending on which colors have become mixed, may also make it completely impossible to reproduce the original colors. Although there is no reference in the above mentioned Japanese Patent Provisional Publication No. 10-136244, let us suppose that the device disclosed therein is used for full-pixel readout by summation of a plurality of lines of the same color. For example, in the case of the aforementioned 5-line summation, there are two possible color combinations on each vertical column, given that the color is in the Bayer arrangement as is true in the case of said patent publication. As it is necessary to read each color pixel individually or read five pixels each of a like color, a total of 10 charge readout electrode systems are required. Therefore, a total of 12 vertical transfer electrode systems are required to perform vertical transfer driving. As explained above, a conventional 3-phase multi-pixel progressive CCD that is capable of 3-phase vertical transfer and omission readout has 4 vertical transfer electrode systems, and performing the line-summation readout necessitates addition of 8 external vertical transfer driving circuits corresponding to the 8 electrode systems, resulting in a considerable increase in the size of the entire CCD drive circuit. Furthermore, should the number of lines to be added increase in response to advances made in the number of pixels on the CCD, the number of electrodes for vertical transfer, too, increases accordingly. Such an increase in number of electrodes for vertical transfer is undesirable because of cost, the size of CCD, power consumption and other considerations.
In cases where shooting is performed in extremely bright conditions, there is the possibility of signal charges overflowing from the pixels to the vertical transfer paths, appearing in the form of streaks on the finished image. Such streaks are called smear and reduce the image quality, presenting a serious problem particularly with a line-summation readout rather than a normal omission readout mode.
Therefore, limiting the modes for capturing moving images to either omission readout or summation readout in the same manner as Japanese Patent Provisional Publication No. 10-210367 may degrade the image quality.
As described above, in cases where a multi-pixel image pickup device is used instead of an image pickup device to improve the image quality of a still image, omission of pixel signals is performed in order to increase the frame rate when shooting moving images. However, a simple procedure of selectively reading out only specific lines while omitting the other lines results in reduced image quality. On the other hand, applying a summation readout method that calls for summing up and using the signal charges of all the pixels to a conventional image pickup device complicates the structure of the image pickup device and increases its cost. Furthermore, depending on the method of summation of pixels, summation readout may produce smear, thereby degrading the image quality.